The invention relates to a method for producing standard gases (CO and H2) for determining the isotope relationships of oxygen and/or hydrogen, in particular during on-line operation, with a sample being decomposed in a (hot) reactor to produce CO and/or H2, and these components being fed to a mass spectrometer, and with the mass spectrometer also being fed with the standard gases for comparison with the gases obtained from the sample. The invention also relates to an apparatus for producing standard gases.
Determination of the isotope composition of hydrogen D/H and oxygen 18O/16O in organic (including water) or inorganic samples is of major importance in hydrology, ecology, foodstuffs chemistry, medicine etc. In order to avoid human errors and to achieve accurate measurement from as many samples as possible in a very short time, the method is carried out during so-called on-line operation. In this case, the samples are placed in a crucible and are introduced into a hot reactor (1450xc2x0 C.) by a so-called auto sampler in accordance with a defined time programme. The reactor contains elementary carbon, and helium flows through it as a carrier gas. The sample is decomposed in the reactor into its constituents, including CO and H2 (assuming these elements were present in the sample). The constituents are then separated from one another in a gas chromatograph, and are analysed successively in a mass spectrometer. An on-line method is described by Saurer et al. in Analytical Chemistry, Vol. 70, No. 10, 1998, pages 2074 to 2080.
The isotope composition of hydrogen and oxygen in the sample is not determined by measuring absolute values. In fact, a comparison is always carried out with so-called standard gases (CO, H2). These standard gases (a defined quantity) must be fed to the mass spectrometer alternately with the constituents obtained from the sample. A number of successive measurements are combined, and are evaluated in order to calculate the isotope composition in the sample relative to the isotope composition in the standard gases. The standard gases are supplied before and after the sample measurement (FIG. 1).
The standard gases are provided in cylinders at a pressure of 200 bar, close to the mass spectrometer. Special precautionary measures are required, since carbon monoxide is extremely toxic, and hydrogen is highly explosive.
The object of the present invention is to provide a method and an apparatus which are safer when used in conjunction with the provision of standard gases.
The method according to the invention is characterized in that the standard gases in the reactor are formed by decomposition, and initial products which are suitable for this purpose are fed to the reactor. It is possible to choose initial products which are safer than CO and H2 and can be decomposed in the reactor into these constituents.
Carbon dioxide (CO2) is advantageously used as an initial product of CO, and/or n-alkanes are used for H2. Carbon dioxide is neither toxic nor explosive. The chosen n-alkanes are preferably those which are less combustible or less explosive, while also being easy to handle, such as propane or butane.
The initial products are decomposed in the reactor to form the standard gases. The latter thus pass, at the latest after the reactor, through the same pipe runs as the constituents obtained from the samples. This reduces fluctuations in the measurement results.
The apparatus according to the invention is naturally particularly suitable for carrying out the method and is characterized by means for feeding initial products for the standard gases into the reactor. As stated above, the standard gases have until now been fed directly to the mass spectrometer. Using the apparatus according to the invention, it is for the first time possible to feed initial products into the reactor to form the standard gases.
The said means for feeding initial products for the standard gases into the reactor advantageously have at least the following components:
a line system,
a connection for at least one initial product,
a connection for a carrier gas supply,
a line leading to the reactor,
a storage line (loop) and
switching means for feeding the initial product into the storage line at times and for likewise carrying the contents of the storage line away at times, with the aid of the carrier gas, into the reactor.
The switching means are preferably constructed on the principle of a rotary valve, such as may be obtained from, for instance, Valco Instruments Co. Inc. (Houston, Tex.), having at least six connections. The connections are connected to one another by means of a common pipeline ring. By operating the valve, it is possible to connect two connections to one another in each case in a first switch position, and to produce two other connection pairs in each case in a second switch position. In this way, an initial product can be fed to the storage line, while the carrier gas is flowing into the reactor. After switching the valve over, the initial product in the storage line is transported into the reactor by the carrier gas, while the initial product connected to the valve is carried away. Taking account of the volumes of the pipeline and the flow rates of the initial product and carrier gas, an initial product can be fed at specific intervals and in suitable amounts to the reactor by switching over the valve for specific intervals.